An ink jet printer of the so-called drop on demand type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead the ink is contained in a plurality of channels in which energy pulses are used to cause the droplets of ink to be expelled, as required, from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the energy pulses are usually produced by resistors, each located in a respective one of the channels and individually addressable by current pulses to heat and vaporize ink in the channels. A thermal energy generator, usually a resistor, is located in each of the channels, a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink thereby forming a bubble which expels an ink droplet. As the bubble grows, the ink which bulges from the nozzles is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink remaining in the channel between the nozzle and the bubble moves towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards the recording medium. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink jet printing is known as drop-on-demand printing. The channel is then refilled by capillary action which, in turn, draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337. The described printer is of the carriage type and has a plurality of printheads each having its own ink supply cartridge mounted on a reciprocating carriage. The nozzles in each printhead are aligned perpendicularly to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped perpendicularly to the line of carriage movement by a distance equal to the width of the printed swath. The carriage is then moved in the reverse direction to print another swath of information. Full width or page width linear arrays, in which the sheet is moved past a linear array of nozzles extending across the full width of the sheet, are also known.
The ink supply cartridge is typically a prepackaged, usually disposable item, having a sealed container holding a supply of ink and having attached thereto a printhead with a linear or matrix array of nozzles. The printhead is either permanently attached to the cartridge, in which case, the entire printhead/cartridge is disposed of, or the cartridge is without a permanent printhead, in which case, the cartridge is disposed of by itself. Generally, the printhead includes terminals to interface with an electronic controller of the printer; electronic parts in the printhead itself are associated with the ink channels in the printhead, such as the resistors and any electronic temperature sensors, as well as digital means for converting incoming signals for imagewise operation of the heaters. Typically, cartridges are purchased as needed by the consumer and used either until the supply of ink is exhausted or until the amount of ink in the cartridge becomes insufficient to maintain the back pressure of the ink to the printhead within the usable range.
While many liquid ink printers have no mechanism to determine the amount of ink remaining within an ink supply, it is desirable to provide an indicator or signal to a user indicating that the supply of ink has been exhausted or has been sufficiently depleted such that continued printing will provide unsatisfactory results. Known ink level sensing systems include providing an indication that the end of the ink supply has been reached, counting the number of ink droplets the printhead is instructed to print, and providing discrete levels of ink level sensing.
Various ink quantity or level sensing systems for determining the amount of ink remaining in an ink supply are illustrated and described in the following disclosures, which may be relevant to certain aspects of the present invention.
In U.S. Pat. No. 4,342,042 to Cruz-Uribe et al., an ink supply system for an ink jet printer including a primary ink supply source and a secondary ink supply reservoir is described. The secondary ink supply reservoir includes a thin flexible membrane which serves as the upper surface and expands or contracts depending upon the amount of ink present in the reservoir. A proximity device senses the movement of the flexible membrane to monitor the quantity of ink present in the reservoir.
U.S. Pat. No. 5,049,898 to Arthur et al., describes a disposable printing assembly including a memory element in which data characterizing the assembly can be stored. The data can characterize the identity of the assembly or one or more of its operational characteristics. A monitoring circuit counts the number of ink droplets the printhead is instructed to print. This count is related to the quantity of ink consumed by the printhead during a given printing task. The memory element on the printhead desirably has a datum thereon that indicates the relative quantity of ink remaining in the ink chamber.
U.S. Pat. No. 5,136,305 to Ims, describes an ink jet printer with ink supply monitoring means. The printer includes a replenishable ink cartridge and ink jet printhead mounted on a carriage for translation across a printing region. The replenishable ink cartridge contains an ink supply monitoring means using a thermistor and constant current circuitry to indicate when the cartridge should be resupplied with ink.
U.S. Pat. No. 5,289,211 to Morandotti et al., describes an ink detecting device which gives advance warning of the end of the ink in a reservoir or cartridge feeding an ink jet thermal printhead. A pair of electrodes that are emersed in a spongy, ink soaked body contained in the reservoir, are arranged in a region adjacent to a feed duct at which the spongy body has a higher capillarity than in remote regions. The electrodes are connected to a bridge circuit which measures the electrical resistance of the ink between the two electrodes.
U.S. Pat. No. 5,386,224 to Deur et al., describes a discrete ink level sensing system including a level sensing probe with at least first and second level sensing pads placed in an ink reservoir. The level sensing system uses electrical conductivity of the ink to detect when the upper surface level of the ink is lower than the lowest points of the level sensing pads.